This invention relates generally to fiber optic communications systems, and, more particularly, to a holographic coupler for coupling, with high efficiency, the output of an optical source, such as a laser diode, into an optical fiber.
The use of fiber optics in communications and data transmission is now rapidly gaining momentum. In fact, several dozen such links have already been installed by major communications corporations. The technology is moving out of the laboratory into the field. The appeal of fiber optic links is largely due to their inherently large bandwidth capabilities, combined with their small size (several orders of magnitude smaller than conventional coaxial cables), and immunity to noise, jamming, and electromagnetic interference.
The laser plays an important part in these systems. Because of its small size and ease of modulation it is typically used as the optical source for the fiber optic cable. These lasers are usually solid state junction lasers whose emitting surface is a stripe typically 1.mu. meter by 10.mu. meters. The optical fibers used have a round geometry, ranging from a few .mu. meters in diameter for monomode fibers, to 100.mu. meters for a multimode optical fiber. Although the dimensions of the laser sources and the fibers are roughly the same, there exists a basic need for an efficient means of coupling the light from the source to the fiber. For example, a recent study of commercially available devices (Electronic Design Magazine, Vol. 28, p. 34, January 1980) showed that the best such devices are only 15% efficient, and the average of such devices were 4.6% efficient. In short, most of the light is wasted. This means that in a long communications link, many more repeater stations are required to compensate for this loss, thus adding to the cost, complexity, while reducing the reliability of the link.
Many of the devices utilized to date are basically lenses. For example, Cohen and Schneider in Applied Optics, January 1974, page 89, describe a micro lens fabricated on the end of the fiber, or the end of the source. An efficiency of 30% is quoted. The major disadvantages of such a device are, because of their extremely small size, the difficulty with which they are fabricated and the hypercritical nature of the alignment between the source and the fiber. For instance, a 3.mu. meter displacement will cause a 50% loss in coupling efficiency, a tolerance difficult to adhere to in the field. Another system proposed by Thyagarajan and set forth in Applied Optics, Vol. 17, 1978 p. 2416, uses a parabolic shaped collector to gather the light and direct it into the fiber. But the same disadvantages set forth above are inherent therein: difficulty of fabrication due to small size and highly critical alignment.
Consequently, there exists a fundamental problem in using a simple lens for this application. A lens is basically an imaging device. It works poorly as an efficient light gathering device. The reasons for this is that in order to gather a large portion of light, the source must be placed close to the lens so that the lens subtends a large angle relative to the source. However, with available refractive indices, such a configuration does not allow the light to be focused. The light gathering ability of a lens is related to the so-called F number of the lens, and F numbers of much less than 1 are not practical. Even a perfect F/1 lens will only collect 20% of the light from a uniform Lambertian point source. Further, if one tries to use a lens as an imaging device, a basic mismatch exists in imaging the stripe geometry of a diode laser onto the round geometry of the optical fiber.
It is therefore readily apparent in the fiber optic field that a need arises for a nonlens-like device which is capable of efficiently and reliably coupling the output of an optical source into an optical fiber.